“We want people to continue to discover and learn about new products and services through Facebook ads without fear of scams or deception," wrote Rob Leathern, product management director for Facebook Business.

"That said, there are many companies who are advertising binary options, ICOs and crypto-currencies that are not currently operating in good faith.”

An ICO - initial coin offering - encourages people to buy into new crypto-currencies before they launch in the hope they are one day worth a lot more money as the coin becomes more popular.

Some companies have used ICOs as a way to raise great sums of money, but without the regulatory burden associated with raising cash though more traditional investment channels.

Because of this, many ICOs have simply disappeared, with little recourse for the "investor" - a loose term, since those who pay into ICOs do not typically get a genuine stake in the new company.

Deception worries

Using Facebook ads to generate "buzz" around a new scheme could be a particularly effective method, especially when paired with celebrity endorsement. Retired boxer Floyd Mayweather was criticised last year for promoting a cryto-currency that later became the target of a lawsuit alleging deception. Mr Mayweather has not been accused personally.

Raising money using an ICO has been banned in South Korea and China, and regulators in other countries are warning against getting involved.

The US Securities and Exchange Commission (SEC) this week said it had seized the assets of a firm that said it had raised $600m in its ICO.

Facebook said its new policy was "intentionally broad" and would evolve over time.

It states: "Ads must not promote financial products and services that are frequently associated with misleading or deceptive promotional practices, such as binary options, initial coin offerings, or crypto-currency."

Under the new policy, advertising using phrases such as "New ICO! Buy tokens at a 15% discount NOW!" or "Use your retirement funds to buy Bitcoin!" would not be permitted, Facebook said.

For Facebook, the move is the latest attempt to protect the integrity of its advertising platform, which over the past 12 months has been under fire for enabling Russian propaganda and other inappropriate content.